Secondary Ion Mass Spectrometry Mode Research Articles

Development of High Throughput Microscope Mode Secondary Ion Mass Spectrometry Imaging

This paper describes the development and initial results from a secondary ion mass spectrometer coupled with microscope mode detection. Stigmatic ion microscope imaging enables us to decouple the primary ion (PI) beam focus from spatial resolution and is a promising route to attaining higher throughput for mass spectrometry imaging (MSI). Using a commercial C60+ PI beam source, we can defocus the PI beam to give uniform intensity across a 2.5 mm2 area. By coupling the beam with a position-sensitive spatial detector, we can achieve mass spectral imaging of positive and negative secondary ions (SIs), which we demonstrate using samples comprising metals and dyes. Our approach involves simultaneous desorption of ions across a large field of view, enabling mass spectral images to be recorded over an area of 2.5 mm2 in a matter of seconds. Our instrument can distinguish spatial features with a resolution of better than 20 μm, and has a mass resolution of >500 at 500 u. There is considerable scope to improve this, and through simulations we estimate the future performance of the instrument.

Sputtering of Oxides from LaNi5 Surface

The changes in chemical composition of the intermetallic alloy LaNi5 surface monolayers were studied using secondary ion mass spectrometry (SIMS) in the process of the alloy interaction with oxygen. The investigated samples were pellets made by pressing the fine-grained LaNi5 alloy. Ar+ ions having energies of 10-18 keV were used as primary ions. The primary beam current density was 9-17 μA·cm-2, which corresponds to the dynamic SIMS mode. The emission intensities of secondary ions were measured within the dynamic range of at least 6 orders of magnitude. Before the measurements, the samples were annealed in residual vacuum at a temperature of ~ 1000 K. After the annealing, the sample surface was cleaned using the primary ion beam until the mass-spectrum composition and secondary ion emission intensity stabilized completely. The gas phase composition was monitored using a gas mass spectrometer. The conducted studies showed that a complex chemical structure including oxygen, lanthanum, and nickel is formed on the surface and in the near-surface region of LaNi5 as a result of its exposure to oxygen. Oxygen forms strong chemical bonds in such a structure with both components of the alloy. This is evidenced by the presence of a large set of oxygen containing emissions of positive and negative secondary ions with lanthanum, with nickel, and oxygen containing lanthanum-nickel cluster secondary ions in mass spectra. The resulting oxide compounds have a bulk structure and occupy dozens of monolayers. In such a bulk oxide structure, the outer monolayers are characterized by the highest ratio of oxygen atom number to the number of matrix atoms. This ratio decreases along the transition from the surface to the underlying monolayers. This process occurs uniformly, without any phase transformation. The observed secondary ions are not a product of association between sputtered surface fragments and oxygen in the gas phase at the fly-off stage after sputter-ejection, but they are products of the oxide compounds being sputtered, hence they characterize the composition of surface and near-surface region.

Co-Registered In Situ Secondary Electron and Mass Spectral Imaging on the Helium Ion Microscope Demonstrated Using Lithium Titanate and Magnesium Oxide Nanoparticles.

The development of a high resolution elemental imaging platform combining coregistered secondary ion mass spectrometry and high resolution secondary electron imaging is reported. The basic instrument setup and operation are discussed and in situ image correlation is demonstrated on a lithium titanate and magnesium oxide nanoparticle mixture. The instrument uses both helium and neon ion beams generated by a gas field ion source to irradiate the sample. Both secondary electrons and secondary ions may be detected. Secondary ion mass spectrometry (SIMS) is performed using an in-house developed double focusing magnetic sector spectrometer with parallel detection. Spatial resolutions of 10 nm have been obtained in SIMS mode. Both the secondary electron and SIMS image data are very surface sensitive and have approximately the same information depth. While the spatial resolutions are approximately a factor of 10 different, switching between the different images modes may be done in situ and extremely rapidly, allowing for simple imaging of the same region of interest and excellent coregistration of data sets. The ability to correlate mass spectral images on the 10 nm scale with secondary electron images on the nanometer scale in situ has the potential to provide a step change in our understanding of nanoscale phenomena in fields from materials science to life science.

Quantitative Surface Analysis of a Binary Drug Mixture—Suppression Effects in the Detection of Sputtered Ions and Post-Ionized Neutrals

A systematic mass spectrometric study of two of the most common analgesic drugs, paracetamol and ibuprofen, is reported. The drugs were studied by means of secondary ion mass spectrometry (SIMS) and secondary neutral mass spectrometry (SNMS) using laser post-ionization (LPI) both in pure samples and in a two-component mixture. Ion suppression within the two-component system observed in SIMS mode is ameliorated using LPI under room temperature analysis. However, suppression effects are apparent in LPI mode on performing the analysis at cryogenic temperatures, which we attribute to changes in the desorption characteristics of sputtered molecules, which influences the subsequent post-ionization efficiency. This suggests different mechanisms of ion suppression in SIMS and LPI modes.

Cluster secondary ion mass spectrometry microscope mode mass spectrometry imaging

Microscope mode imaging for secondary ion mass spectrometry is a technique with the promise of simultaneous high spatial resolution and high-speed imaging of biomolecules from complex surfaces. Technological developments such as new position-sensitive detectors, in combination with polyatomic primary ion sources, are required to exploit the full potential of microscope mode mass spectrometry imaging, i.e. to efficiently push the limits of ultra-high spatial resolution, sample throughput and sensitivity. In this work, a C60 primary source was combined with a commercial mass microscope for microscope mode secondary ion mass spectrometry imaging. The detector setup is a pixelated detector from the Medipix/Timepix family with high-voltage post-acceleration capabilities. The system's mass spectral and imaging performance is tested with various benchmark samples and thin tissue sections. The high secondary ion yield (with respect to 'traditional' monatomic primary ion sources) of the C60 primary ion source and the increased sensitivity of the high voltage detector setup improve microscope mode secondary ion mass spectrometry imaging. The analysis time and the signal-to-noise ratio are improved compared with other microscope mode imaging systems, all at high spatial resolution. We have demonstrated the unique capabilities of a C60 ion microscope with a Timepix detector for high spatial resolution microscope mode secondary ion mass spectrometry imaging.

Microscope mode secondary ion mass spectrometry imaging with a Timepix detector

In-vacuum active pixel detectors enable high sensitivity, highly parallel time- and space-resolved detection of ions from complex surfaces. For the first time, a Timepix detector assembly was combined with a secondary ion mass spectrometer for microscope mode secondary ion mass spectrometry (SIMS) imaging. Time resolved images from various benchmark samples demonstrate the imaging capabilities of the detector system. The main advantages of the active pixel detector are the higher signal-to-noise ratio and parallel acquisition of arrival time and position. Microscope mode SIMS imaging of biomolecules is demonstrated from tissue sections with the Timepix detector.

Mechanisms responsible for inducing and balancing the presence of Cs adatoms in dynamic Cs based SIMS

Secondary ion mass spectrometry (SIMS) is an established technique for sensitive compositional analysis of solids. To achieve high sensitivity in the so-called dynamic SIMS mode, notably in the analysis of negative secondary ions, it is common practice to use Cs primary ions for sputter erosion as well as for loading the sample with Cs. In qualitative terms, the negative-ion yield enhancement has been attributed to a lowering of the sample's work function but, remarkably, the physical processes involved in producing the favorable conditions have not been clarified in any detail before. This study provides evidence that work function changes observed under Cs bombardment can be explained if the implanted ions are transiently converted to adatoms. Previously disregarded properties of Cs atoms include the huge size, the high mobility and bond formation with coverage dependent strength. It is shown that implanted Cs atoms are rapidly relocated towards the receding surface, presumably in response to the stress generated by the retained material. After passage through the solid–vacuum interface, Cs atoms loose their valance electron and become bound to the surface via dipole interaction. This way they become adatoms. Stationary conditions in terms of Cs surface coverage are established by the balance between adatom formation and sputter ejection at energies exceeding the bond strength. Combining calculated sputter cross sections for adatom removal with experimental data from various sources, the rates of Cs implantation and reemission are shown to be in balance with an uncertainty of only about±20%.

Analyses of hydrogen distribution around fatigue crack on type 304 stainless steel using secondary ion mass spectrometry

Secondary Ion Mass Spectrometry (SIMS) analyses were carried out on type 304 austenitic stainless steel. On annealed specimen exposed to hydrogen (10 MPa, 358 K), Element Depth Profiles SIMS mode was able to describe quantitatively the hydrogen profile content computed by the Fick’s law. Based on SIMS analyses on the wake of a fatigue crack (propagation in hydrogen gas at 0.6 MPa and RT), it was possible to compute an apparent diffusivity and solubility in the crack tip region. The apparent solubility and diffusivity in the deformed regions were two times and five orders of magnitude higher than the ones on annealed material, respectively. High hydrogen content was found around the crack tip, where the plastic deformation was well developed (pronounced slip activity). The high apparent diffusivity is presumed to result from enhanced hydrogen transport induced by cyclic plastic activity at the crack tip.

F and Cl detection limits in secondary ion mass spectrometry measurements of Si and SiO2 samples

Usually, F and Cl negative secondary ion currents forming background signals determine detection limits for these elements in secondary ion mass spectrometry (SIMS) measurements. Technique involving changing the primary-ion current was used to determine that the “memory effect” is the main source of the background signal for these elements. It is shown that energy distributions of F and Cl ions forming background signals are distinctly narrower than energy distributions of these ions reflecting element bulk concentration in a Si wafer. Using this fact, the F and Cl detection limits in Si were improved by energy filtering of secondary ions. For experimental conditions used, the filtering of the secondary ion energy in a region around 60–70 eV improves the F and Cl detection limits by a factor of more than 6. The effect of an electron beam used for charge neutralization of insulator samples in a negative SIMS mode on the F and Cl detection limits was also investigated. It is shown that the simultaneous use of the primary ion and electron beams causes a synergistic increase of the background signals. It was determined that both F and Cl detection limits for the SiO2 sample are almost the same as for the Si sample when the electron and ion beams are applied simultaneously. The possible mechanism of this effect is discussed.

Compositional and structural evolution of the titanium dioxide formation by thermal oxidation

Titanium oxide films were prepared by annealing DC magnetron sputtered titanium films in an oxygen ambient. X-ray diffraction (XRD), Auger electron spectroscopy (AES) sputter profiling, MCs+-mode secondary ion mass spectrometry (MCs+-SIMS) and atomic force microscopy (AFM) were employed, respectively, for the structural, compositional and morphological characterization of the obtained films. For temperatures below 875 K, titanium films could not be fully oxidized within one hour. Above that temperature, the completely oxidized films were found to be rutile in structure. Detailed studies on the oxidation process at 925 K were carried out for the understanding of the underlying mechanism of titanium dioxide (TiO2) formation by thermal oxidation. It was demonstrated that the formation of crystalline TiO2 could be divided into a short oxidation stage, followed by crystal forming stage. Relevance of this recognition was further discussed.

A characterization of the β -glycidoxypropyltrimethoxysilane and aluminium interface by SIMS and XPS

Organosilanes are frequently applied as primers to oxide layers on aluminium and its alloys in order to improve adhesion to organic coatings, for example, for corrosion protection, although the bonding mechanism is not well understood [1, 2]. A common view has been that the adhesion involves formation of Si±O±Al type linkages, although direct evidence for this interfacial bonding has proved elusive [3±9]. Earlier work from this laboratory suggested that angle-dependent X-ray photoelectron spectroscopy (ADXPS), in combination with a bias-potential technique, may give a new way for probing the interfacial bonding between oxidized aluminium and silanes, and for aiding the identi®cation of relationships between atomic-level interfacial structure and macroscopic properties [10, 11]. For a thin layer of a-glycidoxypropyltrimethoxysilane (a-GPS) deposited on chemically etched aluminium, it was hypothesized that a new structure apparent in Al2p spectra for small exit angles in ADXPS, after applying a negative biasing potential, may indicate a ` chemical shift'' effect associated with the Si±O± Al bonding [10]. The evidence was indirect; it depended on the existence for some silane±aluminium systems of this additional structure in Al2p spectra, induced by differential charging, and an associated correlation with corrosion protection. This letter reports further observations on the same system, and includes an attempt at characterizing the interfacial bonding with secondary ion mass spectrometry (SIMS) in the static SIMS mode [12]. Following the earlier reports [10, 11], square panels of 7075-T6 aluminium (1 cm) were degreased and acid etched before coating with the silane solution (1 vol % formed by dissolving a-GPS monomer in an equi-volume mixture of distilled water and methanol). Three main samples are referred to below. Sample A was a blank sample prior to coating with aGPS (i.e. the alloy after etching); sample B was formed from sample A by applying the a-GPS solution drop-by-drop until a thick layer of polymer was formed; sample C gave the a-GPS=substrate interface formed by dipping sample A in the a-GPS solution for 5 min followed by air drying. X-ray photoelectron spectroscopy (XPS) spectra were measured with a Leybold MAX200 spectrometer [13] using the unmonochromatized AlKa source (1486.6 eV) operated at 15 kV, 20 mA with pressure in the analysis chamber around 6 3 10y7 Pa. The biased spectra were obtained by applying an external potential of y93 V through the sample holder, and then, after measurement, mathematically shifting the energy scale back by 93 eV [10]. Static SIMS was performed with a VG MM12-12S quadrupole mass spectrometer using a xenon primary beam (5 keV impact energy), for which a current of 0.2 nA irradiated a square area (6.25 mm) on the samples. XPS con®rms that the samples A, B and C had the expected compositions for the surface regions; also sample B was suf®ciently thick that Al signals were not detectable. Sample C showed the extra structure reported previously in the Al2p spectra from using the bias potential technique, and therefore it appeared to provide a suitable sample for probing the a-GPS=aluminium interface with SIMS. Relevant positive ion static SIMS spectra are reported in Fig. 1. Those for the acid-etched alloy (sample A) and the thick a-GPS layer (sample B) serve as references for interpreting that from the interface structure (sample C). The spectrum from sample A in Fig.1a is made up of Al‡ (mass 27) and CH fragments, the latter being indicative of the commonly observed hydrocarbon contamination. Since C2H3 ‡ and C2H5 ‡ fragments are usually comparable in intensity, only part of the mass 27 peak can be assigned to C2H3 ‡. The small peak at mass 28 may be AlH‡ (Si‡ is considered less likely since no Si was detected by XPS). Sample B experienced charging under ion bombardment, and the spectrum in Fig. 1b was obtained with operation of an electron ood gun (500 eV) to compensate. The signals obtained were fairly weak; nonetheless peaks from Si‡ and SiOH‡, and possibly SiCH3 ‡, were evident in the spectrum. The presence of SiOH‡ is consistent with expectation from the a-GPS polymerization [3]. There should be no contribution at mass 27 from Al‡ given the thickness of the a-GPS layer, but hydrocarbon fragments, in part from the a-GPS backbone, are clearly present. Fig. 1c shows the static SIMS spectrum for sample C. No sample charging was evident, thereby supporting the belief that the a-GPS layer was very thin. The most prominent peak was at mass 45 (SiOH‡ from the a-GPS polymerization), but there

Depth profiling free carbon in silicon carbide

A fingerprint for free carbon was identified from the negative ion mass spectra of various carbon and silicon carbide standards. It was found that high‐mass carbon molecular species can be used to distinguish free carbon in silicon carbide. Secondary ion mass spectrometry (SIMS) depth profiling (12 keV Cs+ primary beam, negative SIMS mode) was used to calculate the useful yield of negatively charged molecular carbon ions from carbon (in various forms) and silicon carbide. The useful yield ratio of C−n molecules from carbon to SiC increased with increasing n, meaning higher mass molecular carbon species are more likely to originate from free carbon than silicon carbide. 72C−6 and 96C−8 species were used to depth profile free carbon in chemically vapor deposited SiC/C coatings on SCS‐0 and SCS‐6 silicon carbide fibers.

Depth Profiling of As in an AlAs/GaAs Multilayer by a New Laser-Induced Sputtered Neutral Mass Spectrometry System

Depth profiling of As in an AlAs/GaAs multilayer has been measured with a pulse-counting photoion mass spectrometer (PPMS) that can perform depth profiling in two different modes: a secondary ion mass spectrometry (SIMS) mode and a laser-induced sputtered neutral mass spectrometry (SNMS) mode. In SIMS mode, the As secondary ion intensity in GaAs layers is about three orders of magnitude less than that in AlAs layers under O2+ bombardment even though the As concentration is the same in both layers. In SNMS mode, the As photoion intensity in GaAs is only about twice that in AlAs. This difference can be attributed to sputtering yield differences between GaAs and AlAs. This result suggests that laser-induced SNMS by PPMS will give quantitative information and accurate depth profiling of layered samples by simply correcting sputtering yields.

The realization of a low‐energy ion‐scattering technique with an ion cyclotron resonance spectrometer

AbstractA low energy ion scattering technique for investigation of the first monolayer of a surface has been realized in an ion cyclotron resonance mass spectrometer. The secondary‐ion mass spectrometry mode is also available in the same configuration. Preliminary results are shown for the analysis of the metal surface of a magneto‐optical recording film.

Applications of Sims to Spatially Modified Polymer Film Characterization

AbstractThe predominant application of secondary ion mass spectrometry (SIMS) to organic polymeric solids has been the molecular monolayer analysis of thin films in the “static” mode. The primary emphasis in this paper, however, is the evaluation of SIMS for two or three dimensional compositional mapping studie of spatially modified polymer. This often requires the use of the “dynamic” SIMS mode to provide lateral images, depth profiles, or 3-D image depth profiles. Selected applications are presented including SIMS studies of surface derivatized polymers, metal-doped conductive polymer films, and patterned polymeric materials or fibers. One analytical objective is to assess the extent to which compositional information is limited by primary beam damage. The outlook for SIMS instrumentation combining high lateral spatial resolution with minimal primary beam damage to surface molecules is summarized, for example the combination of microfocused liquid metal ion sources and time-of-flight mass spectrometry.

Applications of SIMS to Spatially Modified Polymer Film Characterization

AbstractThe predominant application of secondary ion mass spectrometry (SIMS) to organic polymeric solids has been the molecular monolayer analysis of thin films in the “static” mode. The primary emphasis in this paper, however, is the evaluation of SIMS for two or three-dimensional compositional mapping studies of spatially modified polymers. This often requires the use of the “dynamic” SIMS mode to provide lateral images, depth profiles, or 3-D image depth profiles. Selected applications are presented including SIMS studies of surface derivatized polymers, metal-doped conductive polymer films, and patterned polymeric materials or fibers. One analytical objective is to assess the extent to which compositional information is limited by primary beam damage. The outlook for SIMS instrumentation combining high lateral spatial resolution with minimal primary beam damage to surface molecules is summarized, for example the combination of microfocused liquid metal ion sources and time-of-flight mass spectrometry.

Coverage dependent structural changes during chlorine adsorption on Ag{110}

The adsorption of Cl2 on Ag{110} at room temperature has been studied using angle-resolved static mode secondary ion mass spectrometry (SIMS), Auger spectroscopy, and LEED. The system has been examined over a coverage range from near zero to the p(2×1) overlayer structure observed by LEED. This structure could be produced by exposing the Ag{110} crystal to 2.5 L of Cl2. The Cl LMM Auger spectra exhibited a distinct change in shape at about 1.0 L exposure which is attributed to a change in the electronic structure of the Ag–Cl bond. In addition, the SIMS Cl− secondary ion yield deviated at this exposure from the expected exponential dependence on work function predicted by theory. An analysis of the secondary Cl− ion kinetic energy distributions, and the polar and azimuthal angle distributions also suggests that the Cl atom is highly charged in the limit of zero coverage with an extended Ag–Cl bond length over that of bulk AgCl. As the coverage approaches the 1.0 L Cl2 exposure point, however, there is significant weakening of surface dipoles due to adlayer interactions. This depolarization appears to be sufficient to allow significant contraction of the Ag–Cl bond length, such that the Cl atom may actually fall into the valley of the {110} surface. A submonolayer Cs overlayer appears to stabilize the negatively charged Cl adatoms, inhibiting the changes observed on the undoped Ag{110} surface. It is suggested that adlayer interactions which influence the formation of ordered overlayer structures mask important electronic and structural features of adsorbates which may only be observed in the single atom limit.

SIMS molecular cluster intensities of inorganic salts containing sulfur and nitrogen oxyanions

Secondary ion mass spectrometry (SIMS) is used to examine alkali sulfur oxyanion and nitrogen oxyanion salts. The secondary ion spectra, filtered about a nominal 1-eV kinetic energy, are measured in the dynamic SIMS mode using a 1—5 μA/cm 2 beam intensity of 1-keV Ar + primary ions. Charge build-up on the sample surface is neutralized by thermal electrons. The positive (+) secondary ion spectra are dominated by cation species (M +, MO +, M 2 +, M 2O +). The negative (−) secondary ion spectra include the anion central atom (A)—oxygen series (O −, A −, AO −, AO 2 −, AO 3 −, AO 4 −). The relative ion intensity pattern of the (−) series shows a clear dependence on anion stoichiometry. For sulfur oxyanions and a 1-keV Ar + ion beam, the pattern can be represented by a modified Poisson distribution if dissociation of AO 4 t- and AO 3 − is assumed for sulfate and sulfite, respectively. The Poisson distribution mean values for oxygen loss are found to be 4̃.5 for sulfate and sulfite and 3̃ for thiosulfate. The mean value for loss increases with increasing Ar + ion energy in the range 0.5–2.0 keV. The nitrogen oxyanion intensifies are complicated by ion beam damage and by neutralization effects. Primary ion beam damage of the substrate is monitored by X-ray photoelectron spectroscopy (XPS).